The present invention relates to a system for a four-wheel drive vehicle for automatically changing the transmission system of the vehicle from two-wheel drive to four-wheel drive in accordance with the inclination of roads.
In a conventional four-wheel drive vehicle, a power transmission system for the two-wheel driving is selectively converted to the four-wheel driving system by engaging a clutch which is manually operated by a select lever.
For example, when the vehicle travels on slippery roads by two-wheel driving, the transmission system should be changed to the four-wheel drive system in order to prevent wheels from slipping. If the changing operation is done after the slipping because of driver's misjudgement of the slipping, the slipping cannot be stopped or reduced.
In order to resolve such a problem, an automatic control system for the four-wheel drive system has been proposed. A conventional system is provided with a slip detecting circuit which produces a clutch signal when the difference between speed of the front and rear wheels exceeds a predetermined reference value. The clutch signal causes a clutch to engage, so that the two-wheel drive is automatically changed to the four-wheel drive.
However, the four-wheel driving is also useful to drive the vehicle uphill and downhill.
Great driving force by the four-wheel driving is effective at uphill driving. In addition, since the engine braking acts on all wheels, the vehicle can be safely driven downhill by the four-wheel driving.